This function ensures that the URL has the correct schema, given the
app configuration. This is required because the Flask instance can sit
behind an SSL-terminating proxy like HAProxy and not know that it is
reachable via HTTPS.
This produces a 'Vary: X-Requested-With' header on the response of
decorated view functions, which indicates to the browser (or intermediate
proxy servers) that the response may/will will be different for XHR and
non-XHR requests.
